Surface plasmon resonance (SPR) is a general spectroscopic method for sensing refractive index changes near the surface of a metal film. Its sensitivity to these changes provides a versatile platform for the observation and quantitation of chemical reactions at the metal/solution interface. SPR allows detection of small changes in refractive index that result from interactions between surface-confined molecules and solution-borne species.
SPR relies on the optical excitation of surface modes (plasmons) in a free electron metal. Back-side, p-polarized illumination of a prism-coupled film at some angle greater than the critical angle for total internal reflection results in plasmon excitation at the metal-solution interface. Plasmon excitation is observed as an increase in optical absorbance (decrease in reflectance) at an optimal coupling angle. This, in turn, results in a minimum in the SPR profile (a plot of reflectance versus angle), which is referred to as the plasmon angle. Sensing via SPR is possible due to the sensitivity of plasmon angle to changes in the index of refraction near the metal surface. Adsorption, desorption, and molecule-molecule interactions that occur at the metal-solution interface result in such changes, thereby inducing a shift in plasmon angle. These changes can be monitored in real-time, making SPR suitable for dynamic sensing.